1. Field of the Invention
The invention relates to a method and an arrangement for monitoring the surface of a structural component for the presence of dew, by interpreting the change occurring in the dielectric constant in the stray field of a condenser, whereby the temperature is measured on such condenser by means of an electric resistance sensor.
2. Prior Art
A number of methods are known in the prior art that can be employed for detecting dew on surfaces. Various physical and chemical effects can be exploited for said purpose. For example, the change occurring in the reflection on the surface of the mirror is used in the presence of dew by means of mirror-type dew point measuring devices with the help of optical evaluation methods. Furthermore, systems are known by which the change in conductivity on electrode structures is interpreted as the measurement effect.
Furthermore, methods are known in connection with which the change occurring in the dielectric electricity constant in the leakage field of a condenser in the presence of dew in the ambient field is interpreted. The arrangements employed for the application of said method offer the advantage that they can be designed in different varieties in a simple manner; manufactured as small systems, and, furthermore, permit simple ways of signal acquisition and signal evaluation.
However, it is a drawback in conjunction with such methods that dirt present on the surface of the measuring arrangement leads to measuring errors. Furthermore, measuring errors may be caused due the fact that a delay occurs in the dew-developing process. This is particularly the case if the passivating layer required for protecting the measuring arrangement has no condensation nuclei.
An arrangement is known according to U.S. Pat. No. 4,626,744, with which soiling of the sensor is detected via changes in the phase position of a measuring ac-voltage occurring as a result of dewing on the sensor.
Furthermore, an arrangement is known according to U.S. Pat. No. 4,948,263 for the determination of the dew point. In said arrangement, two electrodes that are spaced from each by about 50 xcexcm, are provided with a passivation and realized in such a way that the thickness of the insulating layer is smaller than the spacing between the electrodes, and smaller than the thickness of the electrodes as such.
Furthermore, an arrangement is described in DE 41 16 322 A1, by means of which it is possible to determine the composition of the air. The sensors are arranged in said arrangement next to each other in a silicon substrate, and the temperature and the composition of the air are determined with the help of said sensors.
WO 96/05506 describes an arrangement for a dew point sensor that is comprised of a temperature sensor and a condenser, to which an insulating layer consisting of SiO2 and an additional insulating layer consisting of Si3N4 are applied. The arrangement employs a Peltier element for cooling. By evaluating the curve of the signal, the invention permits obtaining information about whether the sensor is dirty.
It is a drawback in conjunction with said arrangements that they require a Peltier element for cooling the system, and that the presence of dewing can be detected only after it has already occurred on the structural component.
It is desirable for many cases of application that when a structural component is cooled, it is possible already shortly before dewing occurs to recognize that a film of water has to be expected on the component soon, so that countermeasures can be initiated. This is the case, for example for preventing glass panes of automobile windows from fogging, or for monitoring electrical installations and sensitive electronic components.
The invention is based on the problem of proposing a method and an array of sensors for detecting condensation on surfaces that makes it possible to generate an evaluation signal already before dewing starts while a structural component is cooling. Said method and arrangement are characterized by low expenditure as well as simple signal evaluation, and avoid errors caused by soiling.
The sensor arrangement as defined by the invention makes it possible to measure the temperature on the capacitor and to employ the temperature sensor at the same time as a heating element in order to eliminate the dewing. With the arrangement as defined by the invention, which is realized in the form of a sandwich-type structure, it becomes possible to produce a detectable film of water on the arrangement already a few degrees of Kelvin before the dew point is reached. By integrating the temperature-dependent resistor in a scatter field capacitor, the temperature is measured directly on the layer where dew is developing, which assures high accuracy.
Another important advantage ensues from the fact that the sensor can be arranged on a flexible material, so that a great variety of application possibilities can be assured in this way.
Areas for the connection sites can be formed on the structured metallic layer in a simple manner, and such areas can serve as bond islands and can be contacted by connection wires.